What is the simple explanation for the terrible support by Firefox and Safari? I figured this was relatively low-hanging fruit, widely used, a big boost for performance (date pickers often load 100s of locales and translations), and a giant move towards sanity for web app developers.

It's simply not a priority. The Bugzilla has been open for 9 years. At one point the assignee of the bug simply stopped logging in and it took 7 months for the autonag to bother people. There simply aren't enough people asking for it. And the fact that it's a Chrome-exclusive feature means it's gonna take a backseat to features that Chrome AND Safari implement but are lacking in Firefox.

There actually was some massive progress on it 3 months ago and it looks like they just need testing now but, again, it's just not a big priority

> a big boost for performance (date pickers often load 100s of locales and translations)

The native date/time pickers work great across all 3 major browsers in my experience. The use-cases for type="week" and type="month" are simply a lot less common.